Adults catch the flu about twice every decade

Influenza infection in adults is actually much less common than some people think(Source: diego_cervo/iStockphoto)

Flu seasonThink about the last time you felt really sick. There's a good chance you may have blamed it on the flu...and there's a good chance you were wrong.

It turns out that adults above the age of 30 actually only catch the flu about two times every 10 years, according to new research published in PLOS Biology .

What can feel like the flu often turns out to be common cold viruses, such as rhinovirus or coronavirus.

On the other hand, the last time your child got sick, there's a better chance it was actually the flu since the same study found children get the flu on average about every other year.

"For adults, we found that influenza infection is actually much less common than some people think. In childhood and adolescence, it's much more common, possibly because we mix more with other people," says Steven Riley, senior author of the study, from the Medical Research Council Centre for Outbreak Analysis and Modelling at Imperial College London.

"The exact frequency of infection will vary depending on background levels of flu and vaccination."

Our immune systems respond to a flu virus by creating antibodies that target the specific viruses' surfaces. We then keep those antibodies in our blood — to be better armed the next time we encounter it.

For the study the researchers from the UK, the United States and China analysed blood samples from volunteers in Southern China.

By looking for virus-specific antibodies in the volunteers' blood samples, the researchers could tell how often each person had encountered the flu. They focused on antibodies to nine known influenza strains that circulated between 1968 and 2009.

The analysis also allowed the team to create a mathematical model for how people's immunity to the flu changes over the years as they encounter different strains of the virus.

The model confirmed earlier studies that have shown our immune response to viruses encountered early in life tend to be stronger than our responses to viruses encountered later in life.

By better understanding our collective immune response to flu viruses, the researchers hope to learn more about how immunity in a population can affect the evolution of flu viruses — which can hopefully lead to stronger immunisations against new strains.